This paper reviews the establishment of a new 50 dry tonne per day biosolids processing facility at a green field site at Nigg Bay, Aberdeen Scotland. Prior to 2001 all sewage had been pumped by long sea outfall into the North Sea. Consequently there was only a minor biosolids operation in the area dealing only with small, rural works. Proposals were sought from a number of consortia for a design, build, finance, operate (DBFO) contract in 1998 by North of Scotland Water Authority (NoSWA) for the Grampian region, which includes new coastal plants at Aberdeen, Peterhead and Fraserburgh as well as many existing smaller plants inland.
NoSWA’s consultant had recommended either thermal drying or incineration for the conforming bid, as there was uncertainty about the agricultural route. A consortium, Aberdeen Environmental Services (AES) including Yorkshire Water, its subsidiary - Grampian Wastewater Services Ltd (GWSL) as operator and EarthTech Engineering Ltd. as designer proposed the CAMBI digestion alternative. The consortium bid was the successful proposal due to a clear financial advantage on capital and operating costs. The inclusion of CAMBI Thermal Hydrolysis was a major contributor to the success of the project.
The advantages of CAMBI for the Aberdeen Project were:
- Absolute guarantee of Class A
- No dryer was required allowing automatic operations, using a single shift
- Digester sizing was 50% of normal.
- Ability to receive and treat a variety of imported sludge
- Net energy production of 1MW electricity
- Simple product to stack and store
- Good grassland product
- Low volume of product (<30,000 tons per year)
- By far the lowest whole life cost
Crucial to the success of the concession was NoSWA’s insistence on the need for a Class A product and the inclusion in the specification of 6 known pathogens that had to be eliminated. Scotland had previously suffered a major E Coli 0157 outbreak with several deaths and this had led to an extreme sensitivity to pathogens. Also, the operator would have to establish a biosolids programme in a region with little history of biosolids use and with a difficult climate. Winters in North of Scotland are long, dark and can be wet or snowy.